For centuries spatial resolution by optical microscopy was believed to be fundamentally limited by diffraction This limit followed from a classical model: gratings with a spacing of less than λ/2 (where λ is the wavelength of the illuminating radiation) will, regardless of the angle of incidence, simply be incapable of scattering light towards a microscope objective. Thus any structural features within an object having a spatial variation smaller than λ/2, will be lost from an image formed by conventional microscopy.
With the development of scanning tunnelling microscopy (STM), resolution was achieved for the first time below the diffraction limit and the family of local probe based microscopes was born. The scanning near-field optical microscope (SNOM, sometimes referred to as NSOM) is a local probe device which detects photons, rather than the electrons of STM.
Models developing the theory of SNOM operation are based on a distinction between radiating (or propagating) and non-radiating (or non-propagating or evanescent) electromagnetic fields. The radiating field is that which is detected by conventional optics (even as close as a distance of a few wavelengths from the sample), that is the field which propagates away from a sample, and which is incapable of communicating sub-wavelength information. The evanescent field is localised at a sample surface and its existence can be deduced from a consideration of the boundary conditions at the interface of a nanometric structure and an illuminating field. This non-radiating field is characterised by high spatial frequencies which reflect surface structure and which are unable to propagate away. In the near-field zone (which term is used herein to refer to the region within which the evanescent field exists around an illuminated sample) both propagating and non-propagating components exist. These are not separable and a perturbation of one will lead to a modification of the other. It was shown by E. Waif and M. Nieto-Vesperinas in “Analycity of the angular spectrum amplitude of scattered fields and some of its consequences”, J. Opt. Soc. Am. Vol. 2, pages 886-889 (1985) that a light beam impinging, on a limited object (where limited in this sense means that the material structure presents a sharp discontinuity) will always be converted into a propagating and an evanescent field. The incident field can be either propagating or evanescent.
The object behind all local probe-based microscopes is to detect an evanescent field formed about a sample via the interaction between the field and a probe (hence local probe). There are a variety of ways in which this has been realised. A review of this field is presented in the paper “Image Formation in Near-Field Optics” by Jean-Jacques Greffet and Remi Carminati in Progress in Surface Science, Vol. 56 (3), pages 133-237 (1997). Examples of local probe SNOM techniques include apertured and apertureless methods, which are each sub-divided into collection and illumination modes. Regardless of data collection technique, a full image of the sample surface is formed by scanning the probe and taking successive data readings.
Upon its development SNOM found many applications. In addition to its obvious relevance to imaging surfaces at a nanometer scale, SNOM has also proven useful in the detection and measurement of confined electromagnetic fields such as surface plasmron polaritons, guided waves and microcavity resonant modes, for local spectroscopy of surfaces; for the modification of surface properties, e.g. nanowriting or modification of magneto-optic domains. This latter application offers great potential for significant advances in high-density data storage.
In apertured SNOM, the most commonly used technique, an aperture with dimensions of tens of nanometers is held within a few nanometers of the surface to be studied. This aperture is usually the end of a sharpened optical fibre, the side surfaces of which are coated in aluminium (to form an opaque “screen” with central aperture). In illumination mode a laser is shone down the optical fibre. As the aperture is sub-wavelength an electromagnetic field cannot propagate and an evanescent field, which decays rapidly with distance, is formed about the probe tip. The evanescent field is scattered and diffracted by the surface under study and this field perturbation is coupled into the propagating field. The propagating waves are then detected in the far field. In collection mode the sample is illuminated in a standard manner, for example by an optical microscope objective, and the apertured probe is again brought to within the near field range of the surface. In this implementation the probe interacts directly with the evanescent and propagating fields present in the near-field zone. The evanescent field itself cannot propagate along the probe, but its interaction with the probe results in the generation of a propagating component which is re-emitted into, for example, an optical fibre light guide
In apertureless SNOM neither detection nor illumination are in the near field. Both are in the far field and the probe is a small scattering tip which is brought into the near field. The probe interacts with the evanescent field generated about the illuminated sample and the results of this interaction are seen in propagating waves collected in the far field. By scanning the probe close to the sample surface therefore, variations in the near field are transferred to the far field. Vertical dithering of the probe and lock-in detection are used in practical instrumentation in order to discriminate signal from background.
Regardless of the detail of the implementation, a key practicality of all local probe microscopes is to find some way of controlling the tip-surface separation in order to ensure that the probe is held within the decay length of the evanescent field either of two methods are commonly employed to achieve this: the “shear force” and photon scanning tunnelling microscopy (PSTM) techniques.
The shear force method involves oscillating horizontally, with respect to the sample surface plane, a vertically-mounted probe, at a frequency close to its resonant frequency. Such an oscillation may be effected by a piezoelectric element vibrating the tip laterally over a few nanometers. As the surface is approached surface-probe interactions lead to a damping of the oscillation amplitude. The damping mechanism under ambient conditions, is generally thought to be due to a confined water layer on the sample surface, but other damping interactions are also feasible. Oscillation amplitude can then be measured, for example, by photovoltaic measurement of an oscillating shadow of the tip in a secondary light beam. By monitoring this amplitude it is possible to maintain it at a constant value and therefore to maintain constant distance between the tip and the sample surface.
The PSTM technique involves monitoring the photon current (by analogy with the electron current in STM) to maintain probe-sample separation. In STM, by adjusting height so as to maintain a constant electron current, the probe can be kept a set distance away from the sample surface. Monitoring the photon current however is far less straightforward. Both evanescent and radiating fields are present in the near field and the detected photon current is not only dependent on the topography of the sample, but also on its material nature and the, distance of analysis. Despite this, photon current can be used effectively in certain specialised circumstances. One such example is when the sample is illuminated by total internal reflection of an incident beam and probed in transmission. In this arrangement illumination is by evanescent field only and so there will be minimal propagating wave on the probe side of the sample. This increases the photon current dependence on topography, and makes monitoring the photon current to maintain separation viable. This mode of SNOM, operating with the combination of illumination by total internal reflection and height control via the photon current, will be referred to herein as PSTM.
A disadvantage of all local probe techniques is data collection time: a full image scan taken with the necessarily small probe is time consuming. Typically, the time taken to collect an image is in the region of several tens of seconds, which precludes real-time monitoring of many scientifically, industrially and physiologically important processes. Furthermore, as local probe techniques are increasingly being used to read and write data beyond the λ/2 limitation of conventional optical storage media, it is rapidly becoming apparent that the speed of data processing is limited by the speed with which information can be read. There is therefore a perceived need to improve data collection times in near-field scanning techniques.